1. Field of the Invention
The present invention relates to a press roll for paper machines, and more particularly to a press roll for use in the press section of a paper machine for removing water from wet paper and making the paper smooth-surfaced.
2. Description of the Prior Art
Roll presses and extended nip presses (ENP) are known as typical means for pressing wet paper for use in the press section of paper machines. The roll press is so adapted that wet paper supported on a felt is passed between two rotary rolls under pressure for the removal of water. With the ENP, wet paper supported on a felt is dewatered by being passed between a rotary roll and a belt to which pressure is applied by a pressure shoe having a large nip width. The rotary roll used in either of these systems has a hard surface in view of the pressing effect and surface smoothness. For example, the roll press comprises the combination of a rotary roll having a hard surface and serving as a top press roll and a rubber-covered roll or the like serving as a bottom press roll.
It is required that such hard-surfaced rotary rolls be usable over a prolonged period of time, withstanding a high load and high-speed rotation. To meet this requirement, stone rolls of natural granite (granite rolls) are widely used. Generally, the stone roll can be mirror-finished over the surface, has high surface hardness, is resistant to abrasion by the doctor blade which is usually provided for removing bits of extraneous stock, permits smooth release of wet paper and is less prone to the deposition of pitch or the like contained in the pulp even when used for a long period. Because of these characteristics, the stone roll has the advantage of being less likely to cause breaks of paper during pressing.
While stone rolls are prepared from natural stone, the stone material is expensive and requires a long period for delivery since the material is difficult to obtain owing to the recent trend toward depletion of resources. In fact, extreme difficulties are encountered in collecting, transporting and processing large stones for making stone rolls which become longer and must be larger than in the past. Further, because the material is a polycrystalline natural stone, there is a substantial problem in that the rolls produced differ in the surface characteristics (such as porosity, surface hardness and water retentivity), even a single roll often differing in such surface characteristics from portion to portion.
In view of the above problems, it has been proposed to replace the stone roll by a synthetic stone roll molded from a mixture of finely divided granite, siliceous sand or the like, and a hard rubber such as NR, NBR, CR or SBR, or a hard resin such as epoxy resin or urethane resin (Unexamined Japanese Patent Publication No. SHO 50-90704, U.S. Pat. No. 2,983,990, etc.). However, the synthetic stone roll has a surface which possesses insufficient water retentivity and poor paper releasability, has low surface hardness and low resistance to the doctor blade due to the use of an organic binder, and is therefore liable to become impaired in smoothness and to permit deposition of pitch. Consequently, the roll is likely to cause breaks of paper in a short period of time and is unfit for long periods of operation. The roll is accordingly often used for pressing wet paper which has been dewatered and given strength by being pressed with a stone roll first.
On the other hand, it has been proposed to use rolls having a ceramic surface layer as conveyor rolls. Ceramic rolls are also proposed which are suited for use in a hot atmosphere as conveyor rolls (Unexamined Japanese Patent Publication No. SHO 58-204884). Nevertheless, it is not known to use ceramics, as proposed by the present invention, for paper machine press rolls which are rotated at a high speed under a heavy load, with water retained in the roll surface.